Defense in Depth to the Desktop
the Strong Internal Network Defense model

Most organizations have expended large amounts of money and resources in
strengthening their perimeter defenses, primarily through firewalls and similar
network hardware mechanisms. Additionally, most organizations rely only on
operating system security controls for the internal networks, not applying
strong internal security controls. The lack of strong internal security
controls is highlighted when the internal network and systems suffer
catastrophic failure when attackers, malware, and, most destructively, worm
viruses make their way into the network inside the defensive perimeter. This
is the classic "eggshell" weakness of network security, hard and crunchy on the
outside, soft and chewy on the inside. The Strong Internal Network Defense
(SIND) model attempts to address this key vulnerability through the application
of hard internal defenses through network hardware.

Overview

Consider the following example of a simplified network. The network is divided
into two subnets; one subnet contains all of the client systems, while the
second subnet contains all of the servers. The client subnet and the server
subnet are separated by a session based, stateful, packet filtering firewall.
The firewall is unidirectional; it only permits traffic that is initiated from
a client to a server. Servers are allowed to reply to clients, but they can
not initiate communication, TCP or UDP, to a client.

Surprisingly, this example does not break Microsoft or most application [*1]
protocols. The result is counterintuitive, but analysis and testing support
this assertion.

In addition to the firewall, the client systems are fully isolated from each
other by layer 2 controls (private vlans). The servers may be similarly
isolated, but doing so is minimally effective and damaging to server to server
communications.

Consider the introduction of a zero day worm virus [*2] into such a network by
an infected client. The client can attack all of the servers, and all of the
servers may become infected. The infected client can not attack any of the
other clients because of the layer 2 isolation. The infected servers can not
attack any of the clients because of the firewall. The end result is that one
client and the servers, a small subset of the organization, are infected. This
is much less devastating, and much easier to clean up, than if the entire
network was infected.

[*1] MAPI, the protocol used by MS Exchange clients (outlook) and the server
has a quirk, acknowledged by Microsoft, affecting new mail notification.
Despite the presence of a perfectly capable TCP connection from client to
server, the server sends a small new mail notification message to the client
from a random high port, UDP, to a dynamic high port on the client. Microsoft
has acknowledged the issue, as highlighted by using clients through a NAT
gateway, but does not give an indication that they care to fix it.

[*2] The infamous zero day worm virus is invoked as a worse case analysis
because it invalidates anti-virus and patch defense mechanisms. Since worms
are increasingly targeting necessary network ports, personal firewalls are also
equally invalidated as a defense mechanism. Marcus can gleefully dance on
their graves.

Analysis

The primary design of the model is to focus security resources on the servers.
No organization can reasonably maintain strict control over client systems, but
they do have absolute control over making sure that servers are currently
patched and running the latest AV signatures. The need to keep client systems
on the patch and AV treadmill is greatly diminished. Client systems can not
directly affect the security of other clients systems, they can only attempt to
harm the servers and themselves.

Application protocols that are broken are peer to peer systems and any kind of
desktop file sharing. This is strongly viewed as a good thing in most
organizations. If I was an attacker going after juicy data the first place I
would look is the poorly secured desktops of the CEO and CFO. Since many
organization appear to be IDS blind on client segments, Id probably fly under
the radar as well.

The model can be easily supplemented with port and protocol restrictions to
further protect the servers from the clients.

The model is very easily scalable, the example is for demonstration purposes.
Research suggests the addition of: a tightly protected master server segment,
for servers that query clients (server -> client protocols, security scanners,
dhcp, backup servers?); a resources segment for things only used by the
servers, like printers; the Internet (duh!); and something intuitive keeps
wanting to separate out authorization servers (Domain Controllers) as well.
Draw circles on the white board for each segment with arrows to model the
firewall, any arrows that point at the clients have the potential to infect the
entire organization, youve been warned.